The melt flow rate (MFR) of a polymer resin is a function of its molecular weight. In general, increasing the melt flow rate allows the resin to be processed at lower temperatures and to fill complex part geometries. Various prior art methods of increasing the melt flow rate involve melt-blending the resin in an extruder with a compound capable of generating free radicals, such as a peroxide. When this is done, the weight average molecular weight of the polymer is reduced and the MFR is increased. Increasing the melt flow rate by decreasing the molecular weight of the polyolefin polymer, however, has been found in many cases to have a detrimental effect on the strength of the modified polymer.
Mestanza et al.—U.S. Pat. No. 6,020,437 disclose a method of improving the rheological properties of polypropylene polymers by melt-blending the polypropylene with (a) a functional compound having at least 2 acrylate groups, (b) a thiuram sulfide compound, and (c) a compound capable of generating free radicals.
Bertin et al.—U.S. Pat. No. 6,620,892 disclose a method of modifying a polypropylene homopolymer or copolymer resin, to increase the melt flow while preserving the strength of the polymer resin, by melt-blending the resin, a stable free radical selected from nitroxyl radicals comprising at least one ═N—O. group, and a peroxide compound (trigger), in the absence of a functional monomer.
Onoi et al.—U.S. Pat. No. 7,019,086, Ashiura et al.—U.S. Pat. No. 7,196,144, and Ashiura et al.—U.S. Pat. No. 7,772,325, all assigned to Yokohama Rubber Co., Ltd., disclose methods to modify an elastomer to improve its bondability, by reacting the elastomer with a compound capable of forming a stable free radical, in the presence of a free radical initiator, such as a peroxide. Examples of such stable free radical compounds include nitroxide radicals, hydrazyl radicals, aryloxy radicals and trityl radicals.
Caronia et al.—US Publication 2007/0145625 disclose a process for cross-linking a polymer after it has been formed into an article. The free-radical crosslinkable polymer is hydrocarbon-based. The free radical cross-linking agent may be selected from (i) hindered amine-derived stable organic free radicals, (ii) iniferters, (iii) organometallic compounds, (iv) aryl azooxy radicals, and (v) nitroso compounds, preferably a bis-TEMPO or 4-hydroxy-TEMPO.
Horst et al.—U.S. Pat. No. 8,618,224 B2 disclose a viscosity breaking process for polypropylene, polypropylene copolymers and polypropylene blends. The vis-breaking of the polymer is conducted, for example, in an extruder, in the presence of an initiator (e.g. peroxide) and a “chain transfer agent.” Suitable chain transfer agents are thiols, disulfides, phosphorous acid esters, phosphines, organic iodides, organic chlorides, propionic acid esters, aldehydes and tertiary amines.
Pham et al.—EP 1 391 482 B1 disclose a polyolefin composition comprising a reactively modified heterophasic copolymer obtained by melt compounding the heterophasic copolymer with an organic peroxide and a bifunctionally unsaturated monomer, such a butadiene.